Problem: $ A = \left[\begin{array}{r}4 \\ -2\end{array}\right]$ $ D = \left[\begin{array}{rr}1 & -2 \\ -1 & -1\end{array}\right]$ Is $ A+ D$ defined?
Explanation: In order for addition of two matrices to be defined, the matrices must have the same dimensions. If $ A$ is of dimension $( m \times  n)$ and $ D$ is of dimension $( p \times  q)$ , then for their sum to be defined: 1. $ m$ (number of rows in $ A$ ) must equal $ p$ (number of rows in $ D$ ) and 2. $ n$ (number of columns in $ A$ ) must equal $ q$ (number of columns in $ D$ Do $ A$ and $ D$ have the same number of rows? Yes Yes No Yes Do $ A$ and $ D$ have the same number of columns? No Yes No No Since $ A$ has different dimensions $(2\times1)$ from $ D$ $(2\times2)$, $ A+ D$ is not defined.